In a method of continuous casting of a steel, a mold powder is added to the molten steel surface in a mold. The powder melts and becomes slag by heat from the molten steel and forms a molten slag layer and flows successively into a gap between the mold and a solidified shell and is thus consumed. The mold powder mainly contains CaO and SiO2 and is also mixed with Al2O3, MgO, Na2O, F, or Li2O in order to adjust the viscosity of the molten slag and the solidification temperature as well as with C in order to adjust the melting speed of the slag. The mold powder has main roles of (A) reliably retaining the lubricating property between the mold and the solidified shell and (B) carrying out moderate cooling by suppressing the heat extraction speed from the solidified shell to the mold.
First, in order to reliably retain the lubricating property between the mold and the solidified shell described in (A), it is important to properly set the viscosity and the solidification temperature in order to ensure the proper flow amount of the molten slag obtained from the mold powder into the gap between the casting mold and the solidified shell. Generally, those with low viscosities are used as the casting speed is higher to assure the flow amount of the molten slag.
The moderate cooling described in (B) is important since it directly affects the surface quality of a slab to be obtained. In the case of steel types such as a hypo-peritectic steel that are easy to cause slab surface cracking, moderate cooling is particularly required. For moderate cooling, it is effective to crystallize crystals in a slag film obtained from the mold powder, particularly on the surface in the mold side. It is because if crystals are crystallized on the surface in the mold side, projections and recessions are formed between the film and the mold and the air layer contained in the projections and recessions works as a heat insulating layer. As the crystals, cuspidine (3CaO-2SiO2—CaF2) is generally used.
However, in the case of producing a slab from a molten steel having a dissolved Al amount of 0.1% or more by a continuous casting method, it becomes difficult to (A) reliably retain the lubricating property and (B) carry out moderate cooling. Because, in the continuous casting of such a high Al steel, SiO2 is consumed by chemical reaction expressed by the following reaction formula (7):4Al+3SiO2→2Al2O3+3Si  (7)Therefore, the basicity [CaO]/[SiO2] in the molten slag is increased and the solidification temperature is considerably increased. Subsequently, a hard sintered substance so-called slag bear is formed in the wall face of the mold to inhibit the flow of the molten slag. As a result, the lubricating property is deteriorated and the solidified shell may stick to the mold to cause breakout.
Further, since the composition fluctuation of the molten slag occurs because of the reaction defined by the above-mentioned formula (7), it becomes difficult to stably produce cuspidine. As described above, in the continuous casting of a high-Al steel, since the composition fluctuation occurs because of the reaction defined by the formula (7), it becomes difficult to stably produce a slab with excellent surface quality.
Therefore, Patent Document 1 proposes a mold powder having a low basicity and a high viscosity as well as a composition and a physical property hard to crystallize to produce a slab excellent in the surface quality even by continuous casting of a high-Al steel, particularly to suppress formation of slag bear (Claims and the paragraphs [0004] to [0007]).
Further, Patent Document 2 discloses a mold powder containing two or more oxides of Group IA elements in the periodic table to produce compounded crystals different from cuspidine and accomplish moderate cooling (Claims and the paragraph [0013]). In this connection, in the invention disclosed in Patent Document 2, LiCa2FSiO4 and NaCa2FSiO4 are disclosed as assumed compounded crystals, and it is supposed that NaCa2FSiO4 is assumed as a main compounded crystal since its Na2O amount is highest among oxides of the Group IA elements in the periodic table used in Examples (the paragraphs [0020] and [0030]). The invention of Patent Document 2 aims to lower the softening temperature of the mold powder and is characterized in that two or more oxides of Group IA elements in the periodic table are contained (the paragraph [0024]).
Patent Document 3 proposes a mold powder having a composition satisfying a prescribed expression of contents of CaO, SiO2, Li2O, F, Na2O, K2O, and Al2O3 and crystallizing cuspidine in a film formed by solidifying a molten layer in order to prevent occurrence of breakout and deterioration of the surface quality of a slab due to increase in solidification temperature and viscosity (Claims and the paragraphs [0011] and [0017]).
However, even in the case of a high-Al steel, particularly a steel having a composition in a range where the peritectic reaction or δ/γ transformation amount is high, even if the above-mentioned mold powder is used, there is a problem that depressions (projections and recessions) and cracking following the transformation shrinkage tend to be caused in the surface of a slab to be obtained. Such a kind of steel is called a hypo-peritectic steel and generally, the chemical component composition range is determined on the basis of the C content [C] in accordance with a binary system equilibrium phase diagram of Fe—C or Fe—Fe2C3. The range is said to be approximately C: 0.09 to 0.18%.
However, in the case of an alloy steel, since the phase diagram itself is changed due to the effect of added elements and both of the maximum solid solution C concentration in a δ phase and the peritectic point are shifted, the composition range of the hypo-peritectic steel cannot definitely be standardized on the basis of solely the C content. Accordingly, regarding the high Al steels, particularly a steel having a composition in which the peritectic reaction or the δ/γ-transformation amount is high, it is known to standardize the following as defined by the expressions (1) to (3) by equilibrium thermodynamics calculation in consideration of the effects of alloying elements such as Si, Mn, Al, Ni, Cr and Mo (Non-Patent Document 1). In addition, with respect to a hypo-peritectic steel to which the following expressions are applied, the contents of the basic components, Si, Mn, Al, Ni, Cr and Mo, are assumed to 4.0% or less (excluding 0%) each and the content of Al is 0.1 to 3.0%.f1−0.10≦[C]≦f2+0.05  (1);f1=0.0828[Si]−0.0195[Mn]+0.07398[Al]−0.04614[Ni]+0.02447[Cr]+0.01851[Mo]+0.090  (2); andf2=0.2187[Si]−0.03291[Mn]+0.2017[Al]−0.06715[Ni]+0.04776[Cr]+0.04601[Mo]+0.173  (3)(In the expressions, [Si], [Mn], [Al], [Ni], [Cr], and [Mo] denote respective contents (% by mass) of Si, Mn, Ni, Cr, and Mo).    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2003-53496 (Claims and the paragraphs [0004] and [0007])    Patent Document 2: JP-A No. 10-216907 (Claims and the paragraphs [0013], [0020], [0024], and [0030])    Patent Document 3: JP-A No. 2002-346708 (Claims and the paragraphs [0011] and [0017])    Non-Patent Document 1: “Solidification”-373 (1985), Solidification Phenomenon Conference 10670, third session, 19th Steelmaking Committee, Japan Society for the Promotion of Science